feat(openai): add /v1/messages endpoint and API compatibility layer

Add Anthropic Messages API support for OpenAI platform groups, enabling
clients using Claude-style /v1/messages format to access OpenAI accounts
through automatic protocol conversion.

- Add apicompat package with type definitions and bidirectional converters
  (Anthropic ↔ Chat, Chat ↔ Responses, Anthropic ↔ Responses)
- Implement /v1/messages endpoint for OpenAI gateway with streaming support
- Add model mapping UI for OpenAI OAuth accounts (whitelist + mapping modes)
- Support prompt caching fields and codex OAuth transforms
- Fix tool call ID conversion for Responses API (fc_ prefix)
- Ensure function_call_output has non-empty output field

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/internal/server/routes/gateway.go

@@ -43,8 +43,28 @@ func RegisterGatewayRoutes(
 	gateway.Use(gin.HandlerFunc(apiKeyAuth))
 	gateway.Use(requireGroupAnthropic)
 	{
-		gateway.POST("/messages", h.Gateway.Messages)
-		gateway.POST("/messages/count_tokens", h.Gateway.CountTokens)
+		// /v1/messages: auto-route based on group platform
+		gateway.POST("/messages", func(c *gin.Context) {
+			if getGroupPlatform(c) == service.PlatformOpenAI {
+				h.OpenAIGateway.Messages(c)
+				return
+			}
+			h.Gateway.Messages(c)
+		})
+		// /v1/messages/count_tokens: OpenAI groups get 404
+		gateway.POST("/messages/count_tokens", func(c *gin.Context) {
+			if getGroupPlatform(c) == service.PlatformOpenAI {
+				c.JSON(http.StatusNotFound, gin.H{
+					"type": "error",
+					"error": gin.H{
+						"type":    "not_found_error",
+						"message": "Token counting is not supported for this platform",
+					},
+				})
+				return
+			}
+			h.Gateway.CountTokens(c)
+		})
 		gateway.GET("/models", h.Gateway.Models)
 		gateway.GET("/usage", h.Gateway.Usage)
 		// OpenAI Responses API
@@ -132,3 +152,12 @@ func RegisterGatewayRoutes(
 	// Sora 媒体代理（签名 URL，无需 API Key）
 	r.GET("/sora/media-signed/*filepath", h.SoraGateway.MediaProxySigned)
 }
+
+// getGroupPlatform extracts the group platform from the API Key stored in context.
+func getGroupPlatform(c *gin.Context) string {
+	apiKey, ok := middleware.GetAPIKeyFromContext(c)
+	if !ok || apiKey.Group == nil {
+		return ""
+	}
+	return apiKey.Group.Platform
+}